1. Field of the Invention
The invention relates to an engine control system for a direct injection-spark ignition type of engine equipped with a lean NOx conversion catalyst in the exhaust line, and, in particular, to a direct injection-spark ignition engine control system which operates the engine with a fuel charge changed between lean and rich.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line a NOx adsorbing type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application WO93/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased, This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
A lean NOx conversion catalyst disposed in an exhaust line as described in the International Patent Application WO93/07363 declines its NOx conversion performance as the temperature of exhaust gas rises. In particular, a NOx adsorption type of lean NOx conversion catalyst shows significant aggravation of NOx conversion performance. In order to keep or enhance the NOx conversion performance, it is desirable to increase combustion velocity of fuel by improving the homogeneity of fuel distribution in the combustion chamber and drop the temperature of exhaust gas as low as possible by rising the combustion efficiency of fuel mixture. However, these demands are not satisfied in the present circumstances.
It is an objective of the invention to provide an engine control system for a direct injection-spark ignition type of engine which rises a combustion velocity of fuel by providing an improved homogeneous distribution of the fuel with an effect of rising combustion efficiency so as to drop the temperature of exhaust gas as low as possibly.
The foregoing object of the present invention are achieved by providing an engine control system operative to control a direct injection-spark ignition type of engine which is equipped with a fuel injector for delivering fuel directly into a combustion chamber and an exhaust system having a lean NOx conversion catalyst for lowering an emission level of nitrogen oxides (NOx) in exhaust gas at an air-fuel ratio of xcex greater than 1 such that the engine makes stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a zone other than the lower engine loading zone. While the engine is monitored to be in a homogeneous charge combustion zone, the engine control system divides a given amount of fuel into two parts, desirably two substantially equal parts, and energizes the fuel injector to intermittently deliver through early and late split injection respectively in a intake stroke and controls the fuel injector to such that a midpoint between points at which said early and late split injection are timed respectively to start is before a midpoint of a intake stroke.
According to the engine control system, while a part of fuel delivered through the early split injection is sufficiently diffused sufficiently homogeneously in the combustion chamber whose volume increases as the piston moves down before the late split injection is started, another part of fuel subsequently delivered through the late split injection is diffused and made homogeneous in the combustion chamber whose volume has sufficiently increased. In consequence, a homogeneous fuel mixture is generated in the combustion chamber with an effect of increasing combustion velocity and combustion efficiency in a combustion stroke, providing a drop in exhaust gas temperature. The control in which fuel injection is made such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke provides various prominent effects described below.
On condition that a given amount of fuel has to be injected within an entire intake stroke, the early split injection can be timed to be caused at a point at which the piston moves down at a relatively high speed, which is accompanied by generation of a strong intake air stream, accelerating accomplishment of a homogeneous distribution of fuel mixture and evaporation of fuel. The early and late split injection is off as one whole to the early side of a intake stroke and, in consequence, fuel delivered through the late split injection sticks to a cylinder wall near when the piston reaches its bottom-dead-center (at the end of a intake stroke), so as to evade tardy accomplishment of a homogeneous distribution of fuel mixture. In addition, there is certainly provided a long period of time allowed for a fuel mixture to evaporate before spark ignition of the fuel mixture and a rise in intake air temperature by recirculated exhaust gas, which are always desirable for acceleration of fuel evaporation. These effects mutually affect one another to produce a greatly improved homogeneous distribution of fuel and significantly improving combustion efficiency of fuel mixture due to a shortened combustion time, lowering the temperature of exhaust gas.
In the zone other than the lower engine loading zone, the engine control system may control the engine to operates with a fuel mixture enriched to be xcex less than 1 while the engine is in a cold condition as well as in a warm condition. Even while the engine is in a cold condition where fuel is slow of evaporation and combustion, the split injection in a intake stroke makes fuel acceleratingly evaporate with an effect of improving combustibility of an enriched fuel mixture. Further, in the zone other than the lower engine loading zone, the engine control system may control the engine to operates with a fuel mixture of xcex greater than 1 while the engine is in a warm condition. This provides an improved homogeneous distribution of fuel sufficient to expand a limit of air-fuel ratio for a lean fuel mixture, lowering specific fuel consumption and the amount of NOx formation. The split injection may be made during an engine start, which is desirable to accelerate evaporation of fuel and prevents the engine from being operated with an overly enriched fuel mixture during a start, so that both startability of the engine and combustibility of lean fuel mixture are realized.
According to another embodiment of the invention, the engine control system further cooperates with an exhaust gas recirculation system to admit exhaust gas into an intake air stream introduced into the engine while the split injection is made. Because of the improved combustibility of fuel due to the split injection, a large amount of exhaust gas can be recirculated into the combustion chamber, providing a drop in pumping loss sufficiently to lower specific fuel consumption and the temperature of exhaust gas. The exhaust gas recirculation rate may be increased up to 10 to 40%.
The fuel injector disposed off a vertical axis of the combustion chamber sprays fuel in a direction intersecting the vertical axis at an angle, so as to make it easy to stratify a fuel mixture with an effect of improving combustibility of the fuel mixture. Further, the engine control system may incorporate an air stream control means operative to create an air stream in the combustion chamber to accelerate accomplishment of a homogeneous distribution of fuel.
According to another embodiment of the invention, the engine control system cooperates with a direct injection-spark ignition type of engine which is equipped with an exhaust system having a lean NOx conversion catalyst for lowering an emission level of nitrogen oxides (NOx) in exhaust gas at an air-fuel ratio of xcex greater than 1 and controls the engine to operate with a fuel charge of xcex greater than 1 in a zone of partial engine loadings. The control system, while the engine is in the partial engine loading zone, divides a given amount of fuel into two parts, desirably two substantially equal parts, which are intermittently delivered through early and late split injection respectively in a intake stroke. The fuel injector is energized such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke.
According to still another embodiment of the invention, the engine control system cooperates with a direct injection-spark ignition type of engine which is equipped with an exhaust system having a lean NOx conversion catalyst for lowering an emission level of nitrogen oxides (NOx) in exhaust gas at an air-fuel ratio of xcex greater than 1 and an exhaust gas recirculation system operative to circulate exhaust gas partly into the intake system from said exhaust system and controls the engine to operate with a lean fuel charge in a zone of partial engine loadings and with a fuel mixture enriched to be xcex greater than 1. The control system, while the engine is in the partial engine loading or lean charge combustion zone under a warm engine condition, divides a given amount of fuel into two parts, desirably two substantially equal parts, which are intermittently delivered through early and late split injection respectively in a intake stroke. The fuel injector is energized such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke, and the exhaust gas recirculation system is caused to admit exhaust gas into an intake air stream introduced into the engine in the partial engine loading zone. When the vehicle proceeds to run under ordinary conditions, the split injection may be made. While the split injection is made, the exhaust gas recirculation system is caused to admit exhaust gas partially into an intake air stream.
In the partial engine loading zone in which the temperature of exhaust gas is generally not risen so high due to an excess of air, the split injection lowers the temperature of exhaust gas even more. Not only during acceleration under which the temperature of exhaust gas tends to rise but during running ordinary conditions under which the temperature of exhaust gas is, if anything, relatively low, the split injection provides a drop in the temperature of exhaust gas as low as possible even during running.
According to a further embodiment of the invention, the engine control system cooperates with a direct injection-spark ignition type of engine which is equipped with an exhaust system having a lean NOx conversion catalyst for lowering an emission level of nitrogen oxides (NOx) in exhaust gas at an air-fuel ratio of xcex greater than 1 and controls the engine to operate with a lean fuel charge in a zone of partial engine loadings and with a fuel mixture enriched to be xcex greater than 1 in a zone other than the lean charge, lower engine loading zone. The control system, when the lean NOx conversion catalyst is monitored to be at a temperature higher than a specified level, divides a given amount of fuel into two parts, desirably two substantially equal parts, which are intermittently delivered through early and late split injection respectively in a intake stroke. The fuel injector is energized such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke.
While the lean NOx conversion catalyst is at a temperature higher than the specified level, the split injection is made to lower the temperature of exhaust gas, preventing the lean NOx conversion catalyst from being overheated.
In the above embodiments, the late split injection is desirably timed to start at a point in one of first and middle divisions of three substantially equal divisions of a intake stroke. Further, the late split injection is desirably timed and to end at a point at in either one of the first and middle divisions while the engine is monitored to operate in a lower engine speed zone. In these instances, the effect of lowering the temperature of exhaust gas is more enhanced. Further, the late split injection may be timed to a point before the midpoint of a intake stroke at which the cylinder piston attains a maximum down speed. In this instance, the piston moving down at its maximum speed creates a strong intake air stream due to which fuel sprayed through the late split injection is diffused and homogeneously distributed in the entire combustion chamber.
The engine control system which, while the engine is in the partial engine loading, divides a given amount of fuel into two parts which are intermittently delivered through early and late split injection respectively in a intake stroke, energizes the fuel injector such that the midpoint between points at which the early and late split injection are timed respectively to start is before the midpoint of a intake stroke can be of course employed for a direct injection-spark ignition type of engine which is equipped with an exhaust system having a catalyst for purifying exhaust gas with an effect of preventing a rise in the temperature of exhaust gas in excess, and hence the temperature of the catalyst. In consequence, the catalyst is prevented from encounters aggravation of catalytic conversion efficiency, durability and reliability of the catalyst.